Load-induced interstitial fluid flow is believed to enhance mass transport in bone to ensure the metabolic function of bone cells and it is also believed to play a role in bone's mechanosensory system via cell shear stresses or cytoskeletal deformations. Understanding how interstitial fluid moves through the bone porosities in normal and diseased states is an important step in elucidating bone's mechanosensory and mass transport processes. We would like to expand our work investigating interstitial fluid flow in cortical bone to study fluid flow in cancellous bone, which has largely been unexamined. We would also like to examine the effect of the metabolic perturbation of osteoporosis on fluid flow in both cortical and cancellous bone. We will do this by applying mechanical loading and observing tracer movement as well as cell and tissue adaptation over time. We will also quantify the fluid space surrounding the osteocytes of normal and osteoporotic bone to provide a much more complete determination of bone's three-dimensional lacunar- canalicular porosity to link microstructure more directly to fluid flow. We propose the following specific aims: (1) Compare the movement of exogenous tracers throughout the cancellous and cortical bone porosities in normal and ovariectomized rats (a) with no applied mechanical loading, (b) immediately after application of mechanical loading, and (c) after a four-week loading experiment where cell and tissue responses are also measured; and (2) Quantify the three-dimensional microstructure of the lacunar-canalicular porosity in normal and ovariectomized rats and use these measurements to calculate bone permeability and interstitial fluid flow in response to mechanical loading. By investigating the links between in vivo interstitial fluid flow, microstructure, and cell and tissue responses to mechanical loading in normal and osteoporotic bone, we believe this work will make a significant contribution to understanding bone's mechanosensory system and may provide insights into the mechanical-related aspects of osteoporosis. This project fits into our long-term goal to better understand the processes of bone adaptation and maintenance. A mechanistic understanding of these important processes should aid in the design of clinical devices or protocols to prevent and treat osteoporosis as well as osteopenia caused by immobilization, bed rest, or a microgravity environment, and should help to improve the design of orthopaedic implants as well as biological bone tissue replacements.